The present invention relates to a data decoding apparatus for reproducing signals from a recording medium such as a magnetic tape, and obtaining video data or audio data.
As means for recording a data row into a magnetic recording medium, and reproducing the recorded signals and decoding into a same data row as before recording, a partial response system (hereinafter called PR) has been known. This is intended to equalize the reproduced signals to have an intersymbol interference, and identify the data row before recording from the signal corresponding to the multilevel. Hereinafter, a data decoding method employing PR (1, 0, -1) of which frequency characteristic is close to the magnetic recording system, as a kind of partial response, is described below.
In a magnetic recording medium, a data row after precoder shown in FIG. 1 is recorded. For example, an input data row . . . 001010 . . . is converted into . . . 001000. . . by the precoder, and recorded. Supposing D to be a delay operation of bit period, the reproduced signal from the magnetic recording medium is reproduced and equalized so that the frequency characteristic through recording and reproducing may be 1-D.sup.2, that is, the level subtracting the signal level of two bits before from the present signal level. It means that the reproduced signal from the magnetic recording medium in which the impulse . . . 001000 . . . may be equalized to . . . 0010-10 . . . .
The signal after being reproduced and equalized is as shown in the eye-pattern in FIG. 2A, in which the reference signal level at the data identification timing is a signal corresponding to three values of -1, 0 and +1. By identifying the signal level corresponding to +1 or -1 to be data "1" and the one corresponding to level 0 to be data "0," the original data row is decoded. Data decoding by partial response is briefly described herein, and the detail is narrated in the reference (Eto, Mita, Doi: Digital video recording technology, Nikkan Kogyo Shimbunsha, pp. 36-38, pp. 46-48).
As shown in FIG. 2A, the opening width of the eye-pattern in the time axis direction in the partial response system is narrower than in the conventional integral detecting method shown in FIG. 2B. Accordingly, due to the effects of the temperature characteristic of the clock reproducing section, if the clock phase showing the detection timing is deviated, the error rate deteriorates significantly. Accordingly, in order to control so that the detection timing may be stably positioned in the center of the opening of the eye-pattern (hereinafter called the eye center), a magnetic recording apparatus is proposed in Japanese Laid-open Patent No. 7-192404. This is intended to detect the phase deviation of the clock from the specific pattern contained in the reproduced digital signal, and control by feedback.
However, the pull-in time of the clock phase of the specific pattern is proportional to the frequency of the specific pattern contained in the reproduced digital signal. Accordingly, in such magnetic recording apparatus, the frequency of occurrence of the specific pattern is enhanced, and the recorded signal is processed by encoding, so that the pull-in time of the clock phase is shortened. In this case, the ratio of the effective data contained in the recorded data is lowered.
Without encoding process for enhancing the frequency of occurrence of the specific pattern, the ratio of the effective data is not lowered, but the pull-in time of the clock phase is extended. As a result, it causes to prolong the reset time to restore the favorable error rate in the discontinuous portion of reproduced signal, such as the head changeover section in the digital VTR or drop-out section derived from the flaw on the recording medium. It is also possible that the specific pattern does not occur for a long period, and the clock phase is deviated in that period, and the error rate deteriorates.
FIG. 6 is an explanatory diagram showing the relation between the detection timing and the distribution of reproduced and equalized signal in the eyepattern. When the initial clock phase is largely deviated from the eye center at time indicated by broken line (a) in FIG. 6, the clock phase may not be pulled into the eye center, and if pulled in, it takes a considerably long time.
The invention is devised to improve the problems of such prior art, and it is hence an object thereof to realize a data decoding apparatus capable of adjusting automatically so that the detection timing of reproduced clock may always come to the eye enter, and obtaining data of low error rate always stably, and not requiring redundancy of recorded data.